1. Field of the Invention
This invention relates to a removable seal and, in particular, to a removable seal which can be positioned in a bore and which can seal, around an element passing through the bore, which, in the preferred example, is a subsea bore such as a riser, BOP package, or well.
In conventional drilling, completion or workover modes, the well fluids are conditioned to allow equipment to access the well under atmospheric pressure. This is achieved by using fluids in the well that exert a over pressure on the open formation which prevents a well influx of fluid from the formation. This method can cause sever damage to the formation and hinder future production rates from a potential reservoir.
An alternative type of operation is to control the well bore pressure in a mechanical manner at a suitable point in the well system which allows the use of optimum types of mud or fluids.
When drilling and operating these wells, either surface or subsea, a two zone pressure regime has to be maintained between the well and the atmosphere. These two zone pressure regime must be maintained even while other downhole operations such as low pressure drilling, completing and testing of the well are carried out, for example, a tubular string passing through a BOP stack on top of the well head.
On a subsea well using a subsea BOP stack, a drilling riser connects the subsea system to the surface vessel. The riser could contain a hydrostatic head of fluid which could provide a undesired pressure effect on the downhole well operations. At different stages of the construction and operation of the well, whether mud, water, oil, foam or gas is being used, the system must be capable of operating with either a high pressure differential in the well relative to the riser or, vice versa, a high pressure in the riser relative to the well. Irrespective of the pressure in the riser, the desired pressure in the well must not be affected.
During the construction and operation of the surface or subsea well, it is often necessary to pass different elements through the various apparatus through the top of the wellhead, for example through the BOP stack, etc. and these elements can include, but are not limited to, drill pipe, tubing strings, casings, wire line, cables and the like. However, due to the movement of these devices, and associated tools and the like which may be mounted on these elements, typically on the lower end, prior known seals which have been utilised to seal the bore through which the elements pass have become damaged and, accordingly, loose their primary function which is to seal the bore around the element passing therethrough.
Typically, the seals which have been previously used are formed as part of the bore itself, for example, as part of a BOP stack or part of a riser section which requires disconnection of the packages to insert a replacement. Accordingly, it is difficult to rectify the damage to the seal, either by repair or replacement, without removing the particular section or apparatus from the well head or associated riser, thereby resulting in significant down time for the well. This results in the operator of the well suffering additional costs whilst the equipment lies idle whilst replacing or repairing a component.
As an example, to seal the BOP bore when drilling, a conventional annular seal is normally used and this allows stripping in and out in coordination with the other annulars. However, as these annulars are part of the BOP package, wear and damaged would require the well to be killed, the damaged BOP package to be withdrawn and replaced by a repaired BOP package. This could typically take 3–6 days in deep water and such a delay is a major disadvantage to the operator of a well.
2. Description of Related Art
U.S. Pat. No. 3,621,912 to L. D. Wooddy, Jr. shows a remotely operated rotating wellhead having a sealing element made of resilient material and a bearing element.
A rotating blowout preventer is disclosed in U.S. Pat. No. 6,129,152 to D. G. Hosie et al. A flexible bladder that is directly activated by a radially disposed pressure chamber to allow for gas tight sealing of a drill pipe therein is shown.
U.S. Patent Application Publication No. US 2001/0050185 A1 to I. D. Calder et al. shows a rotating drilling head with hydraulically inflatable seal for sealing around drill pipe disposed in the drilling head.